N -(3-Fluoro-4-(4-(2-methoxy or 2,3-dichlorophenyl)piperazine-1-yl)butyl) arylcarboxamides as selective dopamine D3 receptor ligands: Critical role of the carboxamide linker for d3 receptor selectivity

N-(3-Fluoro-4-(4-(2,3-dichloro- or 2-methoxyphenyl)piperazine-1-yl)butyl) arylcarboxamides were prepared and evaluated for binding and function at dopamine D3 receptors (D3Rs) and dopamine D2 receptors (D2Rs). In this series, we discovered some of the most D3R selective compounds reported to date (e.g., 8d and 8j, >1000-fold D3R-selective over D2R). In addition, chimeric receptor studies further identified the second extracellular (E2) loop as an important contributor to D3R binding selectivity. Further, compounds lacking the carbonyl group in the amide linker were synthesized, and while these amine-linked analogues bound with similar affinities to the amides at D2R, this modification dramatically reduced binding affinities at D3R by >100-fold (e.g., D3R K _i for 15b = 393 vs for 8j = 2.6 nM), resulting in compounds with significantly reduced D3R selectivity. This study supports a pivotal role for the D3R E2 loop and the carbonyl group in the 4-phenylpiperazine class of compounds and further reveals a point of separation between structure-activity relationships at D3R and D2R.